The present invention relates generally to power amplifier systems, and more particularly to a method and apparatus to increase the linearity of radio frequency power amplifiers by minimizing intermodulation distortion.
Communication services providers, such as cellular system operators, are subject to very strict bandwidth usage spectrum constraints imposed by the Federal Communications Commission (FCC). The FCC licenses transmission channels in the radio frequency spectrum and requires that signals be confined within certain emission limit masks in order to prevent interference caused by signals straying or spilling into adjacent transmission channels. The xe2x80x9cemission maskxe2x80x9d is a power spectrum density envelope. The maximum emitted power allowed varies as a function of the frequency offset from the nominal allocation center frequency. In other words, the emission mask determines the maximum power which may be emitted at a specific frequency for each frequency within the channel allocation. This requires that sideband spillover, the amount of energy outside the licensed channel, be sharply attenuated.
Meeting these emission mask requirements is specially difficult when implementing modern, digitally-based, modulation formats, such as Code Division Multiple Access (CDMA), or Time Division Multiple Access (TDMA). Attenuating the sidebands to meet FCC requirements using such modulation requires very linear signal processing systems and components. Designing linear components, and in particular power amplifiers, at radio frequencies is costly and challenging to achieve.
Radio-frequency (RF) linear power amplifiers (LPA) are typically used in digital cellular base stations to boost the power of a transmitted signal. RF power amplifiers for cellular communications typically operate in the Megahertz (MHz) and Gigahertz (GHz) frequency regions. Boosting a transmitted signal usually requires a LPA with a high ratio of peak-to-average power output (dynamic headroom), typically of at least 10 dB. The challenge is to design LPAs which can provide such dynamic headroom while minimizing sideband spillover without distorting the boosted signal.
Typically, conventional LPA designs have relied on increasing the number of additional RF power transistors in order to achieve the increasingly higher dynamic headroom required by newer implementations. However, using additional RF power transistors has the highly undesirable effect of raising the overall parts count, the manufacturing costs, and the DC current consumption of the LPA.
A fundamental problem in designing linear RF power amplifiers is that power amplifiers are inherently non-linear devices and generate unwanted intermodulation distortion (IMD)
Linearity refers to a characteristic of power amplifiers where there is a substantially constant (linear) gain between an input signal and an output signal. Typically, power amplifiers only exhibit linear gain for a range of input signal voltage levels. This range is often called the linear region of the power amplifier. If the input signal voltage is below the minimum voltage for the linear region or above the maximum voltage for the linear region, then intermodulation distortion occurs.
Intermodulation distortion manifests itself as spurious signals in the amplified RF output signal, separate and distinct from the RF input signal. IMD occurs when different frequencies from the input signal mix to produce sum and difference frequencies which did not exist in the input signal. It is the result of the behavior of amplifier components when operating outside the linear region.
Therefore, a linear RF power amplifier is desired which has substantially linear characteristics, minimizes both sideband spillover and intermodulation distortion, and does not increase the DC current consumption over conventional LPAs.